When applying a steering angle to steered wheels (normally the front wheels except in the case of special vehicles such as a forklift and the like), power steering apparatuses are widely used in order to reduce the force required for the operator to operate the steering wheel. In recent years, as this kind of power steering apparatus, the use of electric power steering apparatuses that use an electric motor as the auxiliary power source are becoming wide spread. Typically, in an electric power steering apparatus, the auxiliary force from an electric motor is applied by way of a reduction gear to a rotating shaft that applies a steering angle to the steered wheels by being rotated by the operation of a steering wheel. Normally, a worm reduction gear is used as this reduction gear. More specifically, by the worm of a worm reduction gear that is rotated and driven by an electric motor engaging with the worm wheel that rotates together with the rotating shaft, the auxiliary power from the electric motor can be transmitted to the rotating shaft. However, in a worm reduction gear, unless certain measures are taken, there are cases in which, due to backlash in the area of engagement between the worm and worm wheel, unpleasant noise called chattering will occur when changing the direction of rotation of the rotating shaft.
In order to suppress the occurrence of this chattering, construction that elastically presses the worm toward the worm wheel has been disclosed in JP 2000-043739 (A), JP 2004-305898 (A) and JP 2006-513906 (A) (WO 2004/074071). FIG. 52 and FIG. 53 illustrate an example of an electric power steering apparatus as disclosed in JP 2004-305898 (A). The front-end section of a steering shaft 2 that is rotated in a specified direction by a steering wheel 1 is supported on the inside of a housing 3 so as to be able to rotate freely, and a worm wheel 4 is fastened to the front-end section of that steering shaft 2. On the other hand, a worm 5 that is rotated and driven by an electric motor 8 is provided in the middle section in the axial direction of a worm shaft 6, and has worm teeth 7 that engage with the worm wheel 4. Both end sections in the axial direction of the worm shaft 6 are supported inside the housing 3 by a pair of rolling bearings 9a, 9b such as deep-groove ball bearings so as to be able to rotate freely. A pressure piece 10 is fitted around a portion of the tip-end section of the worm shaft 6 that protrudes out further than the rolling bearing 9a, and an elastic member such as a coil spring 11 is provided between the pressure piece 10 and the housing 3. The coil spring 11 presses the worm teeth 7 that are provided on the worm shaft 6 toward the worm wheel 4 by way of the pressure piece 10. With this kind of construction, backlash that occurs between the worm teeth 7 and the worm wheel 4 is suppressed, and the occurrence of chattering is reduced.
However, in this conventional construction, it is not possible to suppress chattering that occurs at the joint between the tip-end section of the output shaft 12 of the electric motor 8 and the base end section of the worm shaft 6. In the construction illustrated in the figures, the tip-end section of the output shaft 12 and the base-end section of the worm shaft 6 are joined so that torque can be transmitted by spline engagement between a spline hole 13 that is provided in the base-end section of the worm shaft 6 and a spline shaft section 14 that is provided on the tip-end section of the output shaft 12 of the electric motor 8. As long as the spline hole 13 and the spline shaft section 14 engage with no space between them in the circumferential direction (no backlash), chattering will not occur at the joint section (area of spline engagement) between the tip-end section of the output shaft 12 and the base-end section of the worm shaft 6. However, in actuality, there is backlash in the area of spline engagement. Particularly, in construction for suppressing backlash between the worm teeth 7 and the worm wheel 4, it is necessary to allow for pivotal displacement of the worm shaft 6, so it is not possible to completely eliminate backlash in the joint section between the tip-end section of the output shaft 12 and the base-end section of the worm shaft 6, and thus it is difficult to prevent the occurrence of chattering in the joint section.
As construction for preventing the occurrence of chattering in the joint section between the tip-end section of the output shaft 12 and the base-end section of the worm shaft 6, JP H03-73745 (U) and JP 4,523,721 (B2) disclose construction in which the end section of a driving shaft and the end section of a driven shaft are joined by way of a torque-transmission joint (coupling, shaft joint) that has a shock-absorbing member that is made using an elastic material. FIG. 54 and FIG. 55 illustrate a torque-transmission joint 15 having conventional construction as disclosed in JP H03-73745 (U). The torque-transmission joint 15 has a driving-side transmission member 16 made of metal that is supported so as to be concentric with the tip-end section of the output shaft 12 of an electric motor 8, which is the driving shaft; a driven-side transmission member 17 made of metal that is supported so as to be concentric with the base-end section of a worm shaft 6, which is the driven shaft; a shock-absorbing member 18 made of rubber that is provided between the driving-side transmission member 16 and the driven-side transmission member 17; and a steel ball 19.
The driving-side transmission member 16 has: a circular plate shaped driving-side base section 20 that is supported by the tip-end section of the output shaft 12 such that relative rotation is not possible; and three driving-side arm sections 21 that are provided on the surface of the driving-side base section 20 that faces the driven-side transmission member 17 so as to be intermittently spaced in the circumferential direction and so as to protrude in the axial direction. On the other hand, the driven-side transmission member 17 has a circular plate-shaped driving-side base section 22 that is supported by the base-end section of the worm shaft 6 such that relative rotation is not possible, and three driven-side arm sections 23 that are provided on the surface of the driven-side base section 22 that faces the driving-side transmission member 16 so as to be intermittently spaced in the circumferential direction and so as to protrude in the axial direction. The shock-absorbing member 18 is made using an elastic material such as rubber and has: a hollow cylindrical shaped cylindrical section 24, and six held sections 25 that extend in the radial direction from the outer-circumferential surface of the cylindrical section 24 (on virtual lines that pass through the center axis of the shock-absorbing member 18 and extend in the radial direction). In the assembled state of the torque-transmission joint 15, the driving-side arm sections 21 and the driven-side arm sections 23 are arranged in an alternating manner in the circumferential direction. Moreover, held sections 25 are located between the side surfaces in the circumferential direction of driving-side arm sections 21 and driven-side arm sections 23 that are adjacent in the circumferential direction. Furthermore, the steel ball 19 is held by the tip-end surface of the output shaft 12 and the base-end surface of the worm shaft 6. Therefore, direct contact between the metal driving-side arm sections 21 and the metal driven-side arm sections 23 is prevented, and the occurrence of chattering in this portion is suppressed. During operation, thrust force is transmitted between the output shaft 12 and the worm shaft 6 by way of the steel ball 19, however, thrust force is not transmitted to the shock-absorbing member 18. Therefore, durability of the shock-absorbing member 18 is maintained over a long period of time.
However, in the torque-transmission joint 15, the thrust force that is transmitted between the output shaft 12 and the worm shaft 6 cannot be absorbed by the steel ball 19, so this thrust force cannot be reduced. Therefore, there is a possibility that the thrust force that is transmitted between the output shaft 12 and the worm shaft 6 will become excessively large. Moreover, in forward rotation and reverse rotation of the electric motor 8, thrust forces in opposite directions in the axial direction act on the worm shaft 6, so there is a tendency for the worm shaft 6 to move in the axial direction and become loose, and the steel ball 19 is not able to suppress this looseness of the worm shaft 6. Therefore, there is a possibility that the worm shaft 6 and output shaft 12 will collide with the steel ball 19 with much force, causing noise to occur.
The held sections 25 of the shock-absorbing member 18 are arranged in the radial direction, so in the assembled state of the torque-transmission joint 15, the shock-absorbing member 18 is only exposed to the outside between the driving-side arm sections 21 and driven-side arm sections 23 that are adjacent in the circumferential direction. Therefore, in this torque-transmission joint 15, because it is difficult to visually check the shock-absorbing member 18, there is also a problem in that it becomes easy for a decrease to occur in the work efficiency of the inspection process for preventing assembly of the shock-absorbing member from being forgotten.
Furthermore, in the construction of the torque-transmission joint 15, it is not possible to effectively absorb errors such as dimensional errors or assembly errors of the components of the electric power steering apparatus. For example, when so-called alignment error occurs in which the positional relationship between the center axis of the outer shaft 12 and the center axis of the worm shaft 6 does not coincide, this alignment error is absorbed by elastic deformation of part of the cylindrical section 24 and the held sections 25 of the shock-absorbing member 18. Therefore, the easier it is for elastic deformation of the shock-absorbing member 18 (mainly the cylindrical section 24) to occur, the larger the error is that can be absorbed. However, in the construction of the torque-transmission joint 15, the held sections 25 are arranged in the radial direction, and the driving-side arm sections 21 and driven-side arm sections 23 also are arranged such that the surfaces on the sides in the circumferential direction extend in the radial direction. In other words, virtual planes that include the surfaces on the sides in the circumferential direction of the driving-side arm sections 21 and the driven-side arm sections 23 include the center axes of the driving-side transmission member 16 and the driven-side transmission member 17. Therefore, when the output shaft 12 is rotated and driven and torque begins to be transmitted, a force acts on the held sections 25 that are located between the side surfaces in the circumferential direction on the front side in direction of rotation of the driving-side arm sections 21 and the side surfaces in the circumferential direction on the rear side in the direction of rotation of the driven-side arm sections 23 so as to cause uniform elastic contraction in the circumferential direction from the base-end section to the tip-end section. As a result, a force acts on the cylindrical sections 24 in the pulling direction and it becomes difficult for the cylindrical sections 24 to elastically deform in the radial direction, so together with becoming difficult for the shock-absorbing member 18 to sufficiently absorb the alignment error, surface pressure at the portions of the areas of contact between the outer circumferential surface of the cylindrical section 24 and the inner-circumferential side surfaces of the driving-side arm sections 21 and driven-side arm sections 23 becomes excessive, and there is a possibility that due to increasing the friction resistance in these portions, the transmission efficiency of the overall electric power steering apparatus system will decrease.
In JP 4,779,358 (B2), construction is disclosed in which a shock-absorbing member has three members that are placed together in the axial direction, however, in this construction as well, the held sections of the shock-absorbing member are arranged in the radial direction, so it is difficult to sufficiently absorb alignment error and the like.